In many applications within aircraft, rotating equipment and machinery are soft-mounted to provide benefit in reduced vibration and noise transmission into the aircraft structure. In many aircraft systems, dynamic excitation may occur, related either to failure of the machinery (e.g., rotating equipment), or due to unusual flight conditions or aerodynamic environments. In some instances, resulting low-frequency vibration events may contribute to increased transmission of vibration through attachments of some equipment within the aircraft. This is caused by excitation at or near the fundamental natural frequency of the supported equipment, thereby contributing to resonant response. In these cases, it may be desirable for the soft mounts to stiffen, for example, by changing to a hardened state. This effectively increases the stiffness of the attachment mounts, and shifts the rigid-body natural frequencies of the system away from the excitation frequency. This reduces the dynamic motion of the supported equipment, as well as the loads through the supporting structure and mounts.
One example of vibration caused by a rotating unbalance is a windmilling event. Windmilling is one example of this type of excitation, and occurs when an aircraft fan (e.g., an engine fan, a turbine, or the like) shuts down during flight, for example, upon rupturing one or more fan blades. The fan will continue to rotate, automatically, as an airstream passes therethrough. This occurrence is generally referred to as “windmilling” or a “windmilling event”, as the airstream causes the non-energized fan to freely rotate, similar to a windmill.
Windmilling induces high loads that can propagate throughout aircraft structures and/or systems surrounding the fan. Certification requirements stipulate that support structures, such as struts, rods, bars, and/or beams be designed to carry out (e.g., dissipate or isolate) the high loads for preventing the loads from propagating to adjacent aircraft structures and/or systems. Conventional designs utilize “soft” struts, which include elastomeric materials for reducing shock; however, a problem arises during windmilling events, as the soft struts lack stiffness and thus may increase the transfer of dynamic loads and excitation to adjacent aircraft systems and/or equipment. This induces high displacement and stresses that either cannot be supported by the soft struts, or is damaging to the equipment.
One conventional solution aimed at reducing the high loads transferred in a soft component associated with abnormal high-vibration events includes incorporating snubbing features (e.g., shear pins) within the supporting structures. This is problematic; however, as snubbing features introduce additional vibrations and/or shocks, which are then transferred to adjacent aircraft structures surrounding the fan.
Accordingly, a need exists for improved aircraft support structures, systems, and methods, which are more robust in regards to reducing vibration loads and/or displacements.